Environ. Sci. Technol. 1988, 22, 948-952
Analysis of Atmospheric Ammonia and Particulate Ammonium by a Sensitive Fluorescence Method S. Rapsomanlkis, M. Wake, A.-M. N. Kitto, and Roy M. Harrison" Institute of Aerosol Science, Department of Chemistry, University of Essex, Wivenhoe Park, Colchester C04 3SQ, U.K.
A sensitive fluorescence method for the analysis of atmospheric ammonia and particulate ammonium is described. The fluorescence of the ammonium-o-phthalaldehyde moiety is utilized in a flow injection system to effect a limit of detection of 204 ng m-3 (30) for atmospheric ammonia and 27 ng m-3 (30) for particulate ammonium in 3-h air samples collected respectively on H3P04-impregnatedWhatman 41 and Teflon filters in a filter pack at 10 L min-l. The limit of detection for NH3 may be reduced to 78 ng m-3 (30) by use of H3P04-impregnated nylon filters. An intercomparison with the indophenol blue method is also described together with collection efficiencies on different impregnated filters. The analytical procedure is applicable also to ammonia collected by a denuder coated with phosphorous acid, with the detection limit of 450 ng m-3 (30) for a 3-h sample. A comparison of the filter-pack and denuder sampling methods indicated a small degree of evaporation of ammonium salt aerosol in the filter pack, leading to slight overestimation of ammonia concentrations. Ammonium aerosol concentrations measured by the two techniques were virtually identical since the superior inlet characteristics of the filter-pack sampler compensated for evaporative losses of ammonium salts.
rather tedious for routine use. It is also of limited sensitivity, necessitating rather lengthy sample collection intervals. The aim of the work described here was to develop a sensitive procedure for rapid, routine analysis of solutions of ammonium derived from filter-pack or denuder sampling. The analytical procedure is based upon o-phthalaldehyde-ammonia reaction reported by Roth (13)to form a fluorescent moiety. This has been exploited by Abbas and Tanner (5) in a continuous procedure for analysis of ammonia collected from the atmosphere by a Venturi scrubber but has not to our knowledge been used for analysis of samples of ammonia collected on impregnated filters or of ammonium aerosols. In their report of the method, Abbas and Tanner (5) did not give any experimental details of comparison with other methods in common use. In this study, we have compared the o-phthalaldehyde procedure with analysis by the indophenol blue method. Since reagents used for the coating of denuder surfaces are frequently the same as are used on impregnated filters, our method is also suitable for use with this type of sampler, and we report the use of a phosphorous acid coated denuder. Flow injection analysis provides a rapid and precise means of repetitive analysis and has thus been used in the method described herein.
Introduction With the present focus of attention in atmospheric chemistry upon the problem of acid deposition, interest has blossomed in ammonia as the only significant gaseous atmospheric trace species capable of neutralizing airborne acidity. In this process it converts strong acids to relatively neutral ammonium salt aerosols. The benefit of this partial neutralization is very limited, however, since ammonium salts can be oxidized in the terrestrial environment to form further strong acids (1). Gravenhorst and Bottger (2) have reviewed knowledge of the sources, sinks, and atmospheric cycle of ammonia, highlighting the scarcity of data and imprecision of current quantification of atmospheric fluxes. Ammonia and ammonium salts are thus the subject of considerable current research activity, and there is a clear need for effective, sensitive, and reliable measurement procedures. Atmospheric concentrations of ammonia and ammonium are typically on the order of a few micrograms per cubic meter (3,4) and may be subject to rather rapid temporal variations (5). Although continuous measurement instruments, based upon conversion of ammonia to nitrogen oxide and subsequent detection by chemiluminescence, are available, they are generally of inadequate precision for environmental monitoring at typical atmospheric concentrations. Direct spectroscopic measurement of ammonia is also possible but only at considerable expense over long atmospheric paths (6, 7). A tungstic acid adsorbent has been reported ( 8 , 9 ) ,but it has not been adopted widely for field measurements. Thus, sampling is carried out commonly by filter-pack or denuder methods with subsequent analysis of an integrated sample by wet chemical or instrumental methods (IO). The commonly used indophenol blue procedure (11, 12) is highly reliable but
Experimental Section Analytical Methods. All reagents were obtained commercially (British Drug House Chemicals, Poole, Dorset, U.K.) and used without further purification, unless otherwise stated. Distilled deionized water (DDW) was produced by a Milli-& system. For the analysis of ammonia by fluorescence of the NH4+-o-phthalaldehyde (NH,+-OPA) complex, a flow injection system was set up. A peristaltic pump (P-1, Pharmacia, Uppsala, Sweden) maintained a flow rate of 1.5 cm3 min-l through a 30 cm long reaction loop (Teflon tubing; 1.5-mm i.d.). Samples were injected, after filtration by a micropore disc (Acrodisc, 0.45 pm, Gelman, Northampton, U.K.), into the eluent stream of OPA by a switching valve with a 0.5-cm3 loop (Omnifit, Anachem, Luton, Beds, U.K.). Fluorescence was detected by a high-performance liquid chromatography (HPLC) detector (LDC/Milton Roy, Fluoromonitor 111, Stone, Staffs, U.K.) with a 30-pL cell fitted with a Cd lamp and the appropriate absorption and emission filters. Absorption and emission maxima for the fluorescingspecies occurred at 335 and 455 nm, respectively. The OPA solution containing borate buffer was made up daily in 500 cm3 of DDW and contained 0.08 g of OPA, 9.5 g of disodium tetraborate, 50 FL of mercaptoethanol, and 7.5 g of Brij 35 wetting agent. Analysis of extractant solutions by the indophenol blue method was carried out as follows. In a 25-cm3volumetric flask, 2.5 cm3 of analyte solution was placed together with 5 cm3 of phenol nitroprusside reagent (0.042 M in phenol and 7 X M in nitroprusside) and 5 cm3 of sodium hypochlorite reagent (0.626 M in NaOH and 0.03 M in Clz). After the samples were developed for 30 min at 35-40 O C in a water bath, analysis was carried out by using a spec-
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Environ. Sci. Technol., Vol. 22, No. 6, 1988
0013-936X/88/0922-0948$01.50/0
@ 1988 American Chemical Society
trophotometer (SP6-500; Pye Unicam, Cambridge, U.K.) with a l-cm path length cell at 635 nm. Filter-Pack Sampling. Filter papers (10 at a time, Whatman 41, 47-mm diameter) were washed with approximately 1 dm3 of DDW in a polypropylene Buchner filtration apparatus, impregnated with 5% w/v acid solution (KHS04, citric acid, or H3P04),and dried in a vacuum desiccator. For air sampling, a filter pack consisting of two polycarbonate filter holders (Sartorius SMKB19, Gottingen, FRG) was utilized. The first holder was loaded with a Teflon membrane filter (0.5 pm, Whatman polypropylene backed) for the collection of particulate NH4+and the second with impregnated W41 paper (Whatman 41 cellulose) for collection of gaseous ammonia. After air sampling (3 dm3min-l for 24 h or 10 dm3m i d for 3 h; C. Austen Pumps Ltd., Weybridge, Surrey, U.K.), prefilters were placed in 60-cm3polypropylene bottles and were extracted with 2 cm3of 2-propanol followed by 8 cm3 of DDW. Impregnated W41 filters were extracted with 10 cm3 of DDW. Analysis of the solution by both methods was carried out after extraction during shaking for 15 min on a wrist shaker (Griffin Shaker, Griffin and George Ltd., U.K.). Ammonia filter efficiency studies were carried out by sampling ambient air with a filter pack comprising one Teflon membrane followed by two impregnated filters in series. After extraction and analysis, the collection efficiency was estimated by expressing the ammonia on the first impregnated filter as a percentage of the sum of the two impregnated filters. Denuder Sampling. Air samples were also collected at 10 dm3 m i d by a denuder apparatus consisting of six borosilicate glass tubes in parallel, each 50 cm length X 0.4 cm i.d. The last 35 cm of each tube was coated with phosphorous acid (H3P03),deposited from a 5% solution in methanol. The air from the denuder passed to a poly(tetrafluoroethylene) (PTFE) filter pack containing successively Teflon (0.5 pm), nylon, and H3P04-impregnated filters. After completion of air sampling, the contents of the denuder were extracted into water (30 mL) and analyzed by the OPA fluorescence method. Intercomparison of Filter-Pack a n d Denuder Methods. The denuder sampler described above was operated over 24-h periods at a field site alongside a filter-pack sampler comprising a PTFE filter pack containing the following filters in series: Teflon (0.5 pm), nylon, and H3P04-impregnatedW41. After completion of sampling, the filters and denuder were extracted and analyzed as above. In the filter-pack data, it was assumed that the Teflon filter collected only ammonium aerosols and that all ammonium extracted from the nylon and impregnated W41 filters arose from atmospheric gaseous NH3.
Results and Discussion The motivation for this work was the need to establish and operate a network of 20 sites for sampling and analysis of ammonia and ammonium in air. Expensive sampling apparatus was excluded on the basis of cost, and because of the large number of samples generated, a rapid analytical procedure was essential. The o-phthalaldehyde method described by Abbas and Tanner (5) appeared to offer a suitable analytical procedure but utilized expensive sampling apparatus unsuitable for installation at a large number of sites. We therefore sought to adapt the analytical method for use in the rapid and precise technique of flow injection analysis. Filter-pack methods are widely used for sampling aerosols and gases. For ammonia, they suffer the dual dis-
Table I. Factorial Experiment on Blank W41 Filtersn experiment
factors A
B AB error
sum of squares 9577 52 52 626
A
deg of freedom 1 1 1
4
B
variance
F
9577 52 52 156.5
61.20 0.33 0.33 1.0
nF(1,4)= 21.2; A = washing of filters; B = impregnation of filters; (+) and (-) indicate whether operation A or B has taken place. At the 99% confidence level [F(B) and F(AB) < F(1,4) < F ( A ) ] . Hence operation A is significant to the results.
advantages of possible absorption of ammonia on acidic aerosols collected on the Teflon prefilter and possible volatile loss of ammonium salts from the Teflon filter with subsequent collection as gaseous ammonia. These problems were not felt to be important in our work, since there is little strong acid in locally collected aerosol (14) as evidenced by samples collected with a Teflon filter preceded by an ammonia denuder. Second, if the filter pack is kept at ambient temperature and operates at a low pressure drop, little if any volatile loss of ammonium salts occurs since the ammonium salt aerosols are always in equilibrium with their gaseous precursors (i.e., NH, and HN03 in the case of ",NO3) and evaporation cannot proceed (unlike the case of a Teflon filter which follows a denuder designed to remove one or another of the precursor gases). Thus, as long as the ammonium salts exist in the atmosphere in equilibrium with their dissociation products (i.e., precursor gases), they will be stable when collected on a filter pack. Our previously published experimental data indicate this to be the case (15). Since in other circumstances denuder samplers are more suitable, and many workers prefer to use them, the analytical method was also shown to be effective with air samples collected by phosphorous acid coated denuders. The sensitivity of the fluorescence method is more than adequate for the determination of atmospheric ammonia levels over the short sampling periods described in the present paper. The limit of detection (lod) established with standard solutions of NH4+ is 9 ng for a 500-pL sample loop. The limiting factor for low lod in the filter-pack method is the high-NH4+blanks observed in the impregnated filters. In an effort to reduce filter blanks, a number of variables in their pretreatment were considered. A factorial experimental design (Table I) was used to investigate the effect of two independent variables on the W41 filter blank (16, 17). This design, which also measures the effect of the combination of the variables, requires 22 triplicate experiments for measurement of random error. Estimation of the statistical effect of each variable and variable interaction is carried out by analysis of variance and by comparing variable F-values to the F-value of random error at the 99% confidence level. Variables considered in the present study were washing ( A )and impregnating the filters (B). The effect monitored was the amount of NH4+in the filter blanks. At the 99% confidence level, washing the filters was by far the most significant factor influencing blank values (Table I). Analysis of ammonia by the indophenol blue method has been used extensively (10-12,18) and is regarded as accurate and reliable. Hence, an intercomparison between Environ. Sci. Technol., Vol. 22, No. 8, 1988
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I
I
I
I
Table 11. Collection Efficiencies for Ammonia on Impregnated Filters"
face velocity, m sT1
impregnating reagent
ambient NH3, pg m+
efficiency,
0.26 0.26 0.26 0.33 0.33 0.33
5% KHSOd 5% citric acid 5% 5% KHSOd 590 citric acid 5% H3P04
2.0 2.0 2.0 0.25-0.50 0.25-0.50 0.25-0.50
100 f 2 100 f 3
O n
ov v
I 5
0
I
1
1
I
25
10 15 20 0 P A Fluorescence pg/filter
30
Figure 1. Relationship between the fluorescence and indophenol blue methods of analysis of the same sample. (0)denotes analyses of W41 filters, and (0)denotes analyses of Teflon prefilters.
the fluorescence and the indophenol blue methods was carried out. Samples were collected at a number of different sites over different periods of time and with a variety of flow rates. Exposed impregnated filters and Teflon prefilters (35 in all) were analyzed by both methods simultaneously, and a linear relation was established by plotting the results obtained from one analytical method versus the other (Figure 1). The results from the fluorescence method were plotted on the x axis because of the higher precision of the method (RSD 1-3%; n = 6 ) and the results from the indophenol blue method on the y axis (RSD 8-11%; n = 6). The equation of the line is (NH3)UV= 0.876(NH3)FL+ 0.575 with rz = 0.980 [(NH,) denotes the amount of ammonia per filter in micrograms). To establish that a correlation between the two methods exists, the calculations of the confidence intervals (p value) for the t statistic was employed. For the slope b b 0.876 = -= 7.15 tb = standard error 0.1226 For the intercept a tu =
a
standard error
= - -0'575 0.1226
- 4.69
where standard error = S , / . \ / p . The p value is the probability in the tail beyond the observed values of t. Both observed values of t lie beyond = 3.60 for 33 degrees of freedom. Hence, the probability that the two methods are not correlated is less than 0.0005 ( p value < 0.0005). It can be concluded that there is a good correlation between the two methods and that the value of the intercept is not significant. It is appropriate at this stage to establish whether there is a systematic difference between the two methods, i.e., whether one method systematically indicates higher concentrations than the other or vice versa. The Wilcoxon signed rank test revealed that there is no systematic difference between the two analytical methods at the significance level of p = 0.001. The same test was also applied to results obtained from the Teflon filters only and separately to results obtained from W41 filters. In neither case was a systematic difference between the two methods observed. Interferences in the analysis of ammonia by the fluorescence method described in the present study could originate from the collection of primary amines or amino 950
Environ. Sci. Technol., Vol. 22, No. 8, 1988
%
100 f 2 93 f 5 74 f 8 100 f 2
= 4 in each case.
acids from the atmosphere. Amines, but not amino acids, might be of sufficient volatility to be sampled on the W41 filter, but there was no evidence of systematic error arising from the possible sampling of amino acids associated with particulate matter on the Teflon prefilters or of amines on the impregnated filter (Figure 1). Contamination may occur only during handling of the prefilters and filters, especially when loading them into the filter packs. For this reason, the use of well-cleaned tweezers to manipulate filters and holders is absolutely essential. For collecting ambient gaseous ammonia at different flow rates, a comparison of impregnating acids showed that treatment with a 5% w/v solution of potassium hydrogen sulfate, citric acid, or phosphoric acid was equally efficient (100 f 2%) up to a flow rate of 20 dm3 min-l (filter sampling area 0.00126 m2,linear velocity 0.26 m s-l). At higher flow rates, breakthrough was observed on a second impregnated filter (positioned in series behind the first filter) when potassium hydrogen sulfate or citric acid was being used as the impregnating reagent (Table 11). Filters impregnated with phosphoric acid were found to be efficient (100 f 3%) up to flow rates of 25 dm3 min-l (filter sampling area 0.00126 m2, linear velocity 0.33 m 8). Their efficiency at higher flow rates was not investigated. High filter blanks were the limiting factor in the analysis of atmospheric ammonia. After blanks were reduced by washing (the impregnating reagent was found not to add to the blank values), a low lod could be obtained for modest sampling rates (10 dm3m i d ; 3-h sampling; 1.8 m3 of total sampled volume). For these conditions a lod of 204 ng m-3 of gaseous ammonia (3 u of blank) and because of lower filter blanks, a lod of 27 ng m-3 of particulate ammonium (3a of blank) was achieved. By impregnating nylon filters with H3P04,a detection limit of 78 ng m-3 (3u) for NH3 could be obtained for a 3-h sample due to the lower and more constant blank in these filters. For a sample loop of 0.5 cm3 on the flow injection system, the range of linearity could be extended to 4 mg dm-3 or 40 pg per filter (22.2 pg m-3 for 10 dm3 min-l; 3-h sampling). Some atmospheric ammonium and ammonia concentrations from samples analyzed by the fluorescence method from filter-pack samples are presented in Table I11 and are taken from samples other than those used to generate Figure 1. These are illustrative of the wide range of concentrations encountered in eastern England. Denuder samplers were prepared by coating with 5 % phosphorous acid in methanol. Repetitive determination of the ammonia blank revealed a detection limit of 450 ng m-3 (3u) for a 3-h sample, or 56 ng m-3 for a 24-h sample. Concentrations determined with the denuder are on a similar order to those found with the filter pack; a representative sample of data collected with the two systems operated in parallel appears in Table IV. The mean efficiency of the denuder for ammonia collection was found to be 93.5% by sampling with two denuders in series. This factor has been used to correct the
Table 111. Some NHBand NH4+Values Obtained with t h e Fluorescence Method sampling site
date
sample time
university site (7 samples) Adrigole S.W. Ireland (7 samples) Colchester district (17 sites) university site (8 samples) university site, Colchester (8 samples)
25-31.3.86 10-16.4.86 10-23.4.86 12-13.5.86 14-15.5.86
24 h (3 dm3 min-') 24 h (3 dm3 min-') 7 days (0.8 dm3 min-') 3 h (10 dm3 m i d ) 3 h (10 dm3 m i d )
Table IV. Comparative Field Data from Denuder and Filter-Pack Sampling Procedures (nequiv m-3)
date 28-29.7.87 18-19.8.87 20-21.8.87 14-15.9.87 22-23.9.87 28-29.9.87 8-9.10.87 26-27.11.87 1-2.12.87
ammonia filter denuder pack 82 168 218 137 126 127 71 90 61
100 194 250 133 120 131 51 112 54
ammonium filter denuder pack 182 245 174 55 84 104 22 112 147
220 226 168 50 77 100 19 124 147
results for denuder ammonia in Table IV. The denuder ammonium is the sum of that collected by all the filters following the denuder, after subtraction of the gaseous ammonia estimated to have passed through the denuder. The filter-pack ammonium is that collected by the filterpack Teflon filter, while filter-pack ammonia is all that passes the Teflon filter and is collected by subsequent filters. The ratio of ammonia concentrations by the two methods was calculated for each of the 23 sampling intervals, and the mean (hSD) of the ratios is ammonia (denuder)/ammonia (filter pack) = 0.88 f 0.21 This is strongly suggestive of a small degree of ammonium salt evaporation in the filter-pack sampler with subsequent collection as ammonia. For ammonium, the equivalent ratio for the full data set is ammonium (denuder)/ammonium (filter pack) = 1.00 f 0.12
NH4+,pg m-3 av range 0.63 0.2 2.79 1.09 0.34
0.29489 0.03-0.73 1.45-5.34 0.11-2.47 0.11-0.71
"3,
pg m-3
av
range
5.54 0.91 5.03 0.35 1.45
2.76-11.5 0.52-1.24 0.7-56.8
